Nuclear fueled electric generating plants and thermal renewable fueled electric generating plants of solar thermal and geothermal electric generating plants suffer from periods of partial capacity main steam supply to the primary high pressure steam turbines. It is well known that the primary high-pressure steam turbine energy efficiency suffers significantly at partial capacity or partial load.
Nuclear fueled electric generating plants including both pressurized water reactors (PWR) and boiling water reactors (BWR) are particularly sensitive to this problem because as the nuclear steam supply system, it cannot support sustained full capacity pressure and temperature due to pressure vessel embrittlement and degradation of steam generator tubing. Light water reactor (LWR) nuclear plants run on a low-pressure saturated steam cycle. The wet steam expansion engenders both low turbine efficiency and secondary plant erosion/corrosion problems. The net efficiency for a LWR is typically between 32% and 35%.
The loss of sustained full capacity of the nuclear steam generator is the result of embrittlement of reactor vessel walls within the nuclear steam generator caused by fast neutron fluence damage. Heat annealing of the vessel has been used to partially overcome the embrittlement. Clever core loading can somewhat reduce vessel wall fluence at the cost of having many central core bundles running very near, or at their operational thermal limits. Finally, the most straightforward solution has been to simply reduce the fast neutron fluence exposure of the reactor pressure vessel walls by operating the nuclear steam generator at partial capacity.
Over 400 light water reactors produce approximately 22% of the world's electrical needs, including over 100 in the U.S., about 25 units of VVER-440 in Eastern Europe and Finland that have been in operation since about 1980; 6 units in Russia and 2 units in Ukraine. Many of these reactors are nearing the end of their licensed lifetimes. These plants must either be decommissioned or re-licensed to extend their usable lifetime. In the United States, not even one operating facility has implemented any life extension option beyond partial capacity operation. One reason is that the competitive price of electricity produced with other fuels, specifically natural gas, is difficult to meet with a nuclear fueled electric generating plant. Another related reason is the lower operating temperature and pressure of nuclear fueled electric generating plants reduces thermodynamic efficiency of the Rankine steam cycle through the secondary system of the primary high pressure turbine(s). For example, LWR nuclear plants run on a low-pressure saturated steam cycle. The wet steam expansion engenders both low turbine efficiency and secondary plant erosion/corrosion problems. The net efficiency for a LWR is typically between 32% and 35%.
Attempts at compensating for reduced nuclear steam generator capacity include U.S. Pat. No. 5,457,721 to Tsiklauri et al. wherein a gas turbine provides electrical output and an exhaust gas that passes through a heat recovery boiler to generate superheated steam that flows to a mixer wherein the superheated steam is mixed with the wet main steam from the nuclear fueled steam generator. This approach of mixing superheated steam with wet steam suffers an entropy loss during the mixing. A further disadvantage is that the nuclear fueled steam generator must operate at sufficient capacity to cool the superheated steam to an enthalpy that matches the primary turbine inlet steam requirements. Thus, when the nuclear fueled steam generator falls below a certain capacity or is decommissioned, the superheated steam cannot alone power the primary turbine.
For solar thermal applications, the solar input to a steam generator is only at full capacity during the heat of the day. During the rest of the day, solar input and therefore solar therma steam generator output is at partial capacity requiring either expensive thermal storage or operation of the primary turbine at partial capacity.
For geothermal applications, the steam is generally not superheated and in certain instances flow rate may vary to a partial capacity.
Hence, there is a need for a method and apparatus that would enable full electric generating capacity of the secondary turbine generator portion of aging nuclear power plants as well as full capacity electrical generating capacity of thermal renewable power plants.